178 LIFE OF rocks lying deep in the mines., the glowing mass which Tolcanos eject from the depth of the earth. Hnroboldt does not venture to fix the boundary of the hardened surface and the fluid centre; but he considers that even in these more fluid parts, the movements of ebb and tide dependent on the sun and moon prevail. Now, as experience shows us that heat, in a perpendicular line downwards of 92 Parisian feet, increases by one degree of the decennial Celsius thermometer, granite must exist in a fluid state at a depth of 5*2 geographical miles, four or five times as much as the height of the highest point of the Himalaya mountains. Humboldt distinguishes three modes of the deve- lopment of the inner heat of the earth. The first is, that the strata of earth are periodically wanned and cooled by the sun and the seasons of the year, and thus arises a stream of heat from the exterior inwards, and then again from the interior outwards,* Secondly, in the regions of the equator, a portion of heat penetrates the earth, and flows in it towards the cooler poles,, where it is again united with the air. Finally, our earth has been, since incalculable ages, in a state of gradual refrigeration ; the inner central heat, which originally made the earth glowing hot, loses more and more by the gradual discharge towards the surface and into the atmosphere, although millen- niums do not suffice to measure the degrees. We therefore live, as Humboldt expresses himself, between the glowing heat of the lower strata and the cold atmosphere, of which the temperature is probably "below the freezing-point of quicksilver. (40 degrees of cold of Celsius = 32 degrees of Reaumur.) There are celebrated naturalists who have denied the uninterrupted increase of heat from the surface to * Tliis heat does not penetrate far. In the temperate zones the strata of permanent temperature begins at a depth of fifty-five to sizty feet ; and at half the depth winter and summer warmth have scarcely half a degree influence 011 the Thermometer. In the tropics, the imchangeaHe temperature Ees one 'foot below tfee surface.